The present invention relates to optical thin film filters, and other optical devices requiring aligned dicing and subsequent optical test. More particularly, the present invention relates to dicing a device wafer while affixed to a pre-scribed carrier, and moving the carrier/diced filter assembly through dicing and test fixtures to maintain the positional accuracy of the individual diced devices during dicing and test.
Thin film filters (xe2x80x9cTFFsxe2x80x9d) are manufactured by depositing many, possibly hundreds of layers or films of varying thickness onto a glass substrate. The thickness, refractive index, number and sequence of the layers determine the effect of the filter on incident light, applied in a direction normal to the filter layers. High-pass, low-pass, band-pass and other filter types can be realized.
TFFs are conventionally fabricated using wafer-processing techniques. Each filter may have a planar dimension of 1-2 mm on a side, resulting in thousands of filters on, for example, a 6 inch wafer. Among the difficult issues confronted during fabrication are dicing the wafers post-deposition into individual TFF dies, and then optically testing each die. In its ultimate position in a functioning system, the alignment of a TFF is carefully controlled to be both axially aligned with, and perpendicular to, the incident light source. A similar positioning accuracy must also be attained during post-fabrication optical testing of the TFFs, to ensure their accurate optical characterization.
Other optical devices, such as diffractive optics, micro-lens arrays (small, monolithic, lithographically fabricated lenses), or diffraction grating arrays have similar alignment and test problems.
Currently, the fabrication and test process involves mounting the finished wafer onto a smooth, planar plate; dicing through the wafer and partially into the plate according to a dicing pattern resulting in many individual die; removing the individual TFF die from the plate; affixing each to a carrier; and then cleaning each individual die. Each die thus requires re-collecting into a carrier for subsequent presentation into the test equipment. The test equipment must handle small individual dice and their movement into and out of their test positions, including precision alignment centered on, and perpendicular to, the incident light source.
One major problem with this approach is the need to sort and position each individual die for testing. This process consumes resources including test time and extra equipment. The exposure of the individual dice to this extra handling may also affect the integrity of the dice; there is more likelihood of chipping and scratching (which decreases overall yield).
The process of individual alignment may also lead to testing inaccuracies and/or inconsistencies. There is no sure way to guarantee a consistent test position among individual die since they are individually positioned. Moreover, the small size of each die leads to positioning inaccuracies, especially when aligning the die surface to be perpendicular to the test source. Handling individual die is unproductive as it forces the use of sophisticated machine vision systems, and additional motions, to locate the position of each die, thereby adding considerable cost and complexity, and reducing test throughput.
What is required, therefore, is a technique for fabrication, handling and test of optical devices such as TFFs, which improves process throughput, yield, and test accuracy.
These requirements are met, and further advantages are provided, by the present invention which in one aspect is a technique for fabricating a plurality of thin film filters (xe2x80x9cTFFsxe2x80x9d) from a wafer. The technique includes affixing the wafer to a carrier having a pattern of notches formed thereon corresponding to a pattern into which the wafer is to be diced to form the TFFs. The notches are sized to allow clearance of a dicing apparatus used during dicing.
The wafer is diced at least partially into the notches to form the TFFs, and the TFFs may be individually optically tested with a light source aligned thereto, while they remain affixed to the carrier. The testing includes aligning each TFF to the light source, including moving the carrier relative to the light source, using alignment features pre-placed on the carrier. The carrier is formed from an optically transmissive material, and the optical test includes transmitting light from the light source through the carrier, and in a direction perpendicular to the carrier and filter layers of the TFFs.
Subsequent to testing, the TFFs are removed from the carrier for operation, and the carrier can be re-used for fabricating TFFs from another wafer. To facilitate re-use, a releasable adhesive is applied to the wafer and/or the carrier, and the notches advantageously receive any excess adhesive when the wafer is being affixed to the carrier.
In another aspect, the present invention relates to a carrier for processing a wafer from which a plurality of thin film filters (xe2x80x9cTFFsxe2x80x9d) is to be diced. As discussed above, the carrier includes a planar surface having a pattern of notches corresponding to a pattern into which the wafer is to be diced to form the TFFs. The invention extends to the carrier in combination with the wafer from which the TFFs are to be diced. The carrier is attached to the wafer with a releasable adhesive, and excess adhesive may be present in at least some of the notches. The invention extends to the further combination of the carrier/wafer with a dicing apparatus, the dicing apparatus adapted to use alignment features of the carrier to align to, and dice, the TFFs from the wafer at least partially into the notches.
In yet another aspect, the present invention relates to a carrier in combination with the plurality of diced TFFs, affixed to respective portions of the carrier between the notches with a releasable adhesive. The invention extends to the further combination of the carrier/TFFs with a test apparatus, the test apparatus adapted to use alignment features of the carrier to respectively align to, and optically test, individual ones of the TFFs.
The present invention is also applicable to a range of other optical devices requiring alignment while dicing from a wafer, and a subsequent alignment during optical test.
By using the same carrier during dicing and testing, sub-micron accuracy can be attained when positioning the diced TFFs during optical test. Moreover, the steps of TFF re-mount into individual test carriers, and alignment of each carrier, are eliminated. The present invention therefore greatly increases test accuracy and throughput, and decreases overall processing costs.